Exploring the Degradation Mechanism on PtxCoy Alloy Catalysts for Oxygen Reduction Reaction by Operando X-Ray Absorption Spectroscopy

Abstract

At present, alloying the Pt with 3d-transition metal, such as Co, has been demonstrated as an efficacious approach in enhancing the activity of the cathodic catalysts and diminishing the cost of Pt through ligand effect and strain effect. However, these catalysts were facing with a substantial challenge of the limited durability, primarily attributed to the leaching of the 3d-transition metal under the rigorous electrochemical conditions for the proton exchange membrane fuel cells (PEMFCs)[1]. Herein, we applied both operando conventional XAS and high energy resolution fluorescence detection (HERFD) XAS to identify the electro-chemical behaviors of subsize (about 2.5 nm) PtxCoy alloy catalysts with different Co content and associated with the Pt1Co3@Pt core-shell (CS) nanostructure catalysts obtained by simple displacement reaction. We find that the introduction of Co helps to modify the structure of PtCo alloy catalysts and benefit the activity, however, the increase content of Co brings to the heavier oxidation of Pt, which accelerate the degradation of the PtCo alloy catalysts. While the structure with Pt-rich shell suggested a high tolerance to the Pt oxidation which benefits both stability and activity. Based on these findings, we demonstrate the significance of designing PtCo CS nanostructure with high Co content to achieve optimal performance in PEMFCs. Our research provides valuable insights in designing the catalysts combined with operando XAS analysis and paves the way for the development for the large-scale application for PEMFCs. For the typical synthesis of PtCox alloy catalysts, the Pt(acac)2 and Co(acac)3 were used as Pt and Co precursors. 4mL ethanol was added to the pre-mixed Pt and Co precursors and heated up to 60 °C with magnetic stirring. After the precursors completely dissolved, certain amount of Vulcan 72 was added into the solution. Then repeat ultrasonication and magnetic stirring in 60 °C until the solution converted into slurry statement, dried the product in 30 °C over night. Next, the above well-milled products were treated at 700 °C for 2h in Ar atmosphere to transfer into alloy particles. According to the introduced ratio between Pt and Co, the catalysts were noted as Pt3Co1, Pt1Co1, and Pt1Co3. The contribution of CS nanostructure of Pt1Co3 was applied by simple displacement reaction using H2PtCl6 solution as Pt source. The production was annealed in H2 for 30 min. Operando conventional XAS (at BL36XU in Spring 8) was performed to investigate the electronic statement and structure changes during the operation. Operando HERFD XAS (at BL39XU in Spring 8) further identified the formation of metal Pt with chemisorbed O and Pt oxidation during the polarization varying different Co content and Pt-rich shell. The electronic performance of PtCo alloy catalysts was evaluated in rotating disk electrode, gas diffusion electrode, and membrane electrode assembly, which exhibited a higher performance (about 5 times higher than Pt/C) for Pt1Co3@Pt catalyst. Fig. 1a shows the STEM mapping results for Pt1Co3@Pt catalysts, the linear scanning profile is shown in the mix-image, and the signal intensity between the Pt L-edge and Co K-edge indicates the Pt-rich shell and Pt and Co core structure, which suggested the successful synthesis of Pt-rich shell with high Co content. To further analyze the electronic-behavior for the catalysts, the operando HERFD-XAS analysis was performed on Pt1Co3 and Pt1Co3@Pt catalysts, which provides a higher signal-to-noise ratio and detailed information about the electronic statement of Pt atom[2]. As shown in Figure 1b and c, the main Pt L3-edge white line intensity continuously increased and positively shifted, which suggested the increasing coverage of chemisorbed O or OH onto the Pt surface and the generation of oxidic component at high polarization potential for Pt1Co3 catalyst. While less changes were shown in Pt1Co3@Pt catalyst, which implied the formation of Pt-rich shell could help to suppress the oxidation of Pt in PtCo alloy catalysts and benefit the catalytic performance.